There's no reason it _has_ to be like this. I feel like it became common wisdom that you just shouldn't run a mail server too early and the cool kids all gave up working on mail servers. The author is right though about how terrifying it is that we've ceded running email to an advertising company
All the things that make running your own server hard are software problems, the protocol itself lends itself very well to running your own. We just need someone to build the [Caddy](https://caddyserver.com/) of mail servers. You have the right idea with your Haskell server, though I think in this day a Go project might be more successful.
The "Caddy of mail servers" should be a plugin to Caddy, since server types are pluggable. Caddy's HTTP, DNS, and TCP+UDP servers are all just plugins.
I typically don't recommend someone running their own mail server, but if they do, I wish there'd be a good one they can use, and that one would definitely be a Caddy plugin.
All the things that make running your own server hard are software problems, the protocol itself lends itself very well to running your own. We just need someone to build the [Caddy](https://caddyserver.com/) of mail servers. You have the right idea with your Haskell server, though I think in this day a Go project might be more successful.